The outstanding corrosion resistance afforded by galvanizing has made it the most effective means for the long term protection of steel from oxidation (rusting) and subsequent corrosion. It is the conventional method of providing protection for guardrails, transmission towers, light poles, electrical equipment and dozens of other specific applications. Five mils or 125 microns of a galvanizing composition (applied to light poles and transmission towers) will protect exposed equipment for a period in excess of 20 years. Guardrails may be coated with an average deposition of 75 microns and provide approximately 10 years of corrosion protection in an average rainfall environment. Galvanizing compositions are applied both by hot dipping and electroplating, in applications where surface coatings do not provide adequate corrosion resistance.
Galvanizing compositions are made from zinc "ingots" and become oxidized only when exposed to the elements. The high conductivity of galvanizing compositions provides excellent cathodic protection to steel (which acts as the cathode, zinc being the anode), when exposed to a saline environment or other forms of oxidation caused primarily by water in its various forms, moisture, vapor and ice. Zinc-rich primers have been considered to be optimum anti-corrosion coatings on iron or steel substrates. However, certain problems have restricted their use as industrial type primers. The action of zinc in inhibiting rust is based on an electrochemical interaction between the zinc and the steel substrate. In order not to insulate the zinc particles from each other and from the substrate the prior art has considered it necessary to use very little binder, with satisfactory corrosion protection achieved only when the zinc to binder ratio is at least about 9:1. The high zinc level and the relatively high density of zinc often cause undesirable settling during short term storage, Hence, the zinc is often added just prior to application and mixed rapidly during application to prevent settling and clogging of spray equipment. This deters efficient field use.
A lower zinc content is disclosed in U.S. Pat. No. 3,998,771, issued December 1976 to T. J. Feneis, Jr. et al. Water-based coating compositions are disclosed for application on iron supports to obtain anti-corrosive coatings. Single phase compositions in this patent include about 2% to 10% by weight of a non-volatile liquid epoxy resin, with low viscosity, derived from bisphenol A and an epihalohydrin, e.g., epichlorohydrin; about 2% to 10% by weight of a modified polyamide, i.e., an addition product of a water soluble polyamide and a liquid epoxy resin; and about 55% to 70% by weight of a zinc pigment having an average particle size of about 2 to 15 microns.
U.S. Pat. No. 4,417,007, issued November 1983 to G. A. Salensky et al, discloses a one component composition containing from about 4% to 25% by weight epoxy or phenoxy resin binder and a polyamine hardener, about 43% to 90% by weight zinc dust, about 3% to 38% by weight Mn.sub.3 O.sub.4 fume pigment, up to 35% by weight additional pigments including pigment extenders and fillers (such as talc, clays, diatomaceous silica and silica), up to 5% by weight pigment suspension agent (such as hydrous magnesium silicate and lecithin), and balance organic solvents. A 1:1 volume ratio of zinc dust to Mn.sub.3 O.sub.4 is preferred.
U.S. Pat. No. 4,891,394, issued January 1990 to the applicant of the present invention, discloses a coating composition for the protection of metallic and non-metallic substrates against environmental attack, comprising about 10% to about 25% by weight of a film-forming polymer which may be epoxy resins having an epoxide value of about 250 to 2500, vinyl chloride resins copolymerized with polyisocyanates, and/or vinyl chloride resins copolymerized with melamines; about 30% to about 60% by weight particulate metallic zinc; an agent for control of electrical conductive characteristics comprising a crystalline silica having an oil absorption value of less than 20 as measured by ASTM Test D281-84, the volumetric ratio of such agent to the metallic zinc ranging from about 0.7:1 to about 1.25:1; about 2% to about 3% by weight of an agent for control of rheological characteristics comprising a pyrogenic amorphous silica having an average particle size less than about 0.012 micron; and at least one solvent compatible with the polymer.
French patent application 2,602,239, published Feb. 19, 1988 in the name of the applicant of the present invention, discloses a two phase coating composition containing up to 70% by weight of a powdered metal (based on the total weight of the composition after admixture), from about 2% to 30% of a film-forming polymer, about 2% to about 30% of a hardener for the polymer, at least 1.8% to 30% of an agent for control of rheological characteristics, and up to 30% by weight organic solvents. The preferred polymer is an epoxy resin having an average molecular weight of 350 to 3800. The agent for control of rheological characteristics comprises at least one pyrogenic silica and optionally at least one natural or transformation silica having an oil absorption value preferably not greater than 90 and more preferably not greater than 40. In the specific examples, pyrogenic silicas were used having average particle sizes of about 0.014 micron, about 0.007 micron and about 0.008 micron.
U.S. Pat. No. 4,748,194, issued May 1987 to Geeck, discloses a coating composition for the protection of gas tanks, comprising a powder metal (such as zinc, cadmium, stainless steel, aluminum, alloys or mixtures thereof), a linear epoxy or phenoxy resin having a molecular weight of less than 15,000 cross-linked with a blocked isocyanate, a suspension agent, a thixotropic agent, and "active" and "inactive" organic solvents. The proportion of powdered metal present ranges from 13 to 52 parts per hundred. The suspension agent disclosed in this patent is polyethylene, and the thixotropic agent is silane treated silicon dioxide, in amounts up to 2 parts per hundred.
U.S, Pat. No. 4,621,024, issued Nov. 4, 1986 to F. A. Wright, discloses metal coated microspheres and a process for preparation of the microspheres. Particulate zinc, aluminum, silver, copper, stainless steel, platinum, gold, or mixtures thereof, having an average particle size of about 6 to 10 microns, are bonded to the surfaces of non-conductive microspheres by means of a thermosetting adhesive coating on the microspheres with application of heat, followed by intermittent mixing in the absence of heat. The microspheres may be fly ash, comprising about 80%-96% by weight alumina-silica, with minor amounts of iron oxide, alkaline earth metal oxides and alkali metal oxides. The adhesive binder preferably comprises an organo-functional silane and a copolymerizable monomer. In the final product the metal is from about 15 % to about 30% by weight, relative to the weight of the adhesive binder-coated microspheres, Although this patent discloses average particle size diameters of metal coated microspheres ranging from about 60 to 180 microns, the assignee of this patent also produces zinc coated microspheres of smaller average diameters, e.g., about 2.5 to about 60 microns,
The use of zinc-coated microspheres disclosed in the above mentioned U.S. Pat. No. 4,621,024 in zinc-rich inorganic binder compositions has been proposed by the prior art, as a partial replacement for zinc dust. More specifically, substitution of between 20% and 40% by volume of zinc-coated microspheres, for the zinc dust, has been evaluated in a silicate primer (produced by Carboline Company of St. Louis, Mo., under the trademark "Carbo Zinc 11"). Silicate binders of this type have a very slow drying time, and also require blast cleaning of the metal substrate prior to deposition of the coating. These coatings are electrically conductive.
U.S. Pat. No. 5,182,318 ("the '318 patent"), by the applicant of the present invention, discloses a coating composition exhibiting improved resistance to corrosion of metallic substrates. The glass microspheres in the '318 patent are coated with zinc and are concentrated primarily at the exposed surface of a dry coating, so as to provide corrosion protection to the substrate. The '318 patent provides suitable conditions (by solvent selection), for the hollow zinc-coated glass microspheres to rise to the surface of the coating, to provide the desired protection to the substrate.
Glass microspheres, not coated with a metal, heretofore have been used primarily in non-paint related uses such as: polymeric panels which form a part of airplanes; syntactic foams, electrical potting compounds, randomes in the aerospace industry; syntactic foams in the hydrospace industry; plastisols, adhesives, polymeric spare parts in the automotive industry; wall repair compounds, caulks, sealants and tape joint compounds in the construction industry; in increasing the velocity of detonation, optimum sensitization and chemical stability of industrial explosives; as part of sporting goods such as tennis rackets, flyfishing lines, bowling balls and golf balls; as trowling mix and putty for the marine market; and other applications.
In all of the above listed uses, some of the characteristics of "non-coated" microspheres which are of greatest significance are: the lightness (weight) and resulting lower composite density; spherical shape; inherent strength because of the sphericity, as compared to other fillers; cost effectiveness due to the lower composite density (cost is even lower for the "non-coated" microspheres compared to the metal-coated microspheres); chemical resistance; excellent moisture resistance; low dielectric constant; low electric conductivity; decreased application and drying time, etc.
Contrary to the above listed uses, U.S. Pat. No. 5,252,632 ("the '632 patent") discloses coating compositions, comprising lightweight, non coated hollow, glass microspheres having high isostatic crush resistance, low specific gravity and low oil absorption, for use as cathodic coatings for prevention of corrosion in metallic substrates; and for use as conductive coatings for attenuation of EMI/RFI interference in electronic components, the conductive coatings being suitable for application on metallic and non-metallic substrates.
Co-pending patent application 07/972,115 ("the '115 application, a continuation of the '115 application has been filed on Jan. 23, 1995"), now abandoned by the applicant of the present invention, discloses an improved low-cost coating composition for use in non-gloss and low-gloss applications which require high, dry deposition thicknesses, said composition comprising, apart from the film-forming polymer and the volatile components, from about 5 to 30 volume percent of lightweight, hollow, glass microspheres, having diameters ranging from 1 to 150 microns.
As demonstrated by the '632 patent and the '115 application, many of the above properties exhibited by these microspheres could be beneficially used in coatings-related applications. However, manufacturers such as Minnesota Mining (3M) and PQ Corporation, make it clear that their sales of the "non-coated", hollow, glass microspheres in the past 12 years to the coatings industry have been insignificant, at best. The principal reason for this lack of enthusiasm for the lightweight microspheres is that their low specific gravity (specific gravity generally ranges from 0.1 to 0.6), causes the spheres to float to the surface (caking), making commercial exploitation rather difficult, except where this particular property is specifically desired and appropriately exploited, as in the '632 patent.
For a cathodic coating, it is important that the final coating itself be a good conductor (have a conductivity of at most 3 ohms/cm.sup.2). Conventional wisdom would therefore point away from using the "non-coated" microspheres in cathodic coatings, because of their low electrical conductivity.
As in the '632 patent and the '115 application, the present invention solves the "lightness of the spheres" problem with the use of (1) appropriate wetting agents; (2) appropriate dispersants;(3) appropriate chemical thickeners; and (4) glass microspheres with specific gravity greater than water. It is believed that the low electrical conductivity problem is overcome by (a) the inherent tendency of the microspheres to occupy a much smaller volume within the coating, as compared to conventional extender pigments and (b) the naturally low oil absorption displayed by the microspheres. It is believed that these two characteristics of the "non-coated" hollow glass microspheres, allow the free movement of metal in a liquid phase between the spheres, thereby not restricting the overall conductivity of the coating.
As discussed above, the use of "non-coated", hollow, glass microspheres, instead of conventional pigments, in conductive coatings, results in unaffected overall electrical conductivity; lower overall specific gravity of the coating; improved tensile strength, flexibility, corrosion resistance, water-vapor impermeability; and lower chances of "mud-cracking".
U.S. Pat. No. 5,098,938, issued March 1992 to R. R. Savin, discloses a coating composition similar to that of the above-mentioned U.S. Pat. No. 4,891,394, wherein an epoxy resin film-forming binder is used, and wherein at least four different size grades of pyrogenic amorphous silicas are present within specified proportions and average particle sizes, together with a crystalline silica having an oil absorption value of less than 20 measured by ASTM Test D281-84.
Canadian Patent 2,065,828 provides a waterborne zinc-rich anticorrosion primer which is based upon the combination of metallic zinc dust with a stable aqueous dispersion of a particular chlorinated addition copolymer. Such primer can be formulated without the need for significant amounts of organic co-solvents. There primers readily cure at ambient temperatures, allow overcoating shortly after drying, and result in films of desirable hardness, resiliency and adhesion both to the substrate and topcoat.
Canadian Patent 2,074,329 relates to an improved powder coating composition comprising (a) a resin, (b) a curing agent and (c) zinc, wherein the zinc is a mixture of (c1) lameliar zinc (zinc flakes) and (c2) zinc dust.
U.S. Pat. No. 5,167,701 issued December 1992 to R. R. Savin discloses a one-package zinc-rich coating composition having an inorganic binder which provides protection of metallic substrates against environmental attack comprises, in volume percent: from about 55% to about 60% of an alkyl silicate solution having a solids content of about 35% to about 45% by weight; about 10% to about 14% zinc dust of at least one different particle size grade; about 0.5% to about 2.5% zinc flakes; about 3% to about 6% particulate ferrophosphate; about 10% to about 17% of a particulate crystalline silica having an oil absorption value of less than 20 measured by ASTM Test D 281-84; about 1% to about 2.5% of at least two different size grades of pyrogenic amorphous silicas having average particle sizes ranging from about 0.007 to about 0.04 micron; about 0.3% to about 0.5% of a wetting agent; and about 7% to about 8% of an anhydrous alcohol solvent.
Zinc powder has been used only rarely in zinc rich coating systems due to its large particle size, heavy sedimentation problems and has been ignored as an acceptable pigment. This application involves the use of zinc powder as a low cost galvanized metal replacement. Its relatively larger particle size permits excellent topcoat adhesion while galvanizing generally requires pre treatment such as acid etching or special wash primers to provide adequate adhesion. Most galvanized metal is normally not coated due to the cost involved in the pre-treatment and the application of a topcoat in field conditions. While the term "zinc powder" has been and continues to be used interchangeably with "zinc dust", as used herein "zinc powder" only means metallic zinc in granular form, which is different from "zinc dust", from "zinc powder" and from "particulate zinc", as these terms have sometimes been used in the prior art. As used herein "zinc powder" also is different from "lameliar zinc" or "zinc flakes", as used in the Canadian Patent 2,074,329.
Coatings made from zinc dust provide only limited protection to bare metal due to its much lower conductivity than zinc metal caused by oxidation during its manufacturing process. In conventional zinc rich paints, the greater the conductivity the greater the area of adjacent bare steel that will be protected by the zinc metal. The level of adjacent bare metal protection is largely proportional to its conductivity measured in ohms/cm.sup.2. All galvanizing compositions, prior to exposure, will measure total conductivity of 0.00 ohm/cm.sup.2 at 75 micron deposition, whereas zinc rich industrial end maintenance coatings will measure from 1 to several dozen ohms/cm.sup.2 at 75 microns based on the percentage of zinc dust and the particle size of the zinc dust utilized. In order to provide adequate cathodic continuity, zinc incorporated in primers customarily contains between 80-95 weight % of zinc dust to the binder including additives. The high percentage of zinc dust provides improved conductivity contributing improved cathodic protection, however, the high density and low binder content causes serious problems in handling and poor substrate adhesion.
Another consideration is that inorganic zinc rich coating compositions are difficult to manufacture and store because of the irreversible curing mechanism triggered by exposure to moisture. Once the zinc dust has been added to a one package zinc primer in the manufacturing process, the moisture curing mechanism is activated. Exposure to a minimum relative humidity of 50% effectively cures these inorganic zinc rich coatings. Any exposure to moisture in the manufacturing or packaging process will destabilize the primer by premature curing, resulting in a product that will gel in the container in about two days. These coatings have therefore been manufactured in inert environments, using nitrogen blankets to prevent moisture contamination. One package longer shelf life zinc primer coatings have been marketed in small volumes, since the nitrogen blanketing equipment necessary to produce such coatings in a stable form, is specialized and expensive.
Despite the sensitivity to moisture, inorganic zinc rich coating compositions have been suggested for topcoating to prevent environmental attack.
Zinc powder has rarely been used in zinc rich coating systems and has been ignored as an acceptable pigment due to its large particle size and heavy sedimentation problems. This application involves the use of zinc powder in a stable coating composition as a low cost galvanized metal replacement. Its relatively larger particle size permits excellent topcoat adhesion, while galvanizing generally requires pre-treatment such as acid etching or special wash primers to provide adequate adhesion. Most galvanized metal is normally not coated due to the cost involved in the pretreatment and the application of a topcoat in field conditions.
U.S. Pat. No. 5,338,348 ("the '348 patent") discloses a coating composition for use in protecting metallic substrates from corrosion, comprising in weight percent, based on the total weight of the composition: from about 7% to 35% of film-forming substance; from about 35% to 55% of zinc powder; from about 5% to 25% of zinc flakes; from about 1% to 5% at least one kind of amorphous silica; and up to about 30% particulate ferrophosphate. No microspheres, zinc or glass, are utilized in the compositions of this invention.
Co-pending U.S. application Ser. No. 08/287,913 (of which this application is a continuation-in-part application), discloses a stable coating composition for use in protecting metallic substrates from corrosion, comprising in weight percent, based on the total weight of the composition: from about 7% to 35% of alkyl silicate as a film forming substance; from about 35% to 55% of zinc powder; from about 5% to 25% of zinc flakes; from about 0.2 to 5% of at least one amorphous silica; and up to about 30% particulate ferrophosphate; wherein the alkyl silicate comprises, in weight percent, based on the weight of the alkyl silicate: from about 5% to 20% of tetraethyl orthosilicate.
As described above, previous patents and patent applications of the applicant of the present invention have made two separate advancements in the field of cathodic coatings: (1) the successful utilization of uncoated glass microspheres in such coatings with the benefits associated with the same and without the potential detriments which had led persons skilled in the art away from utilizing the microspheres; and (2) the successful incorporation of zinc powder (as defined above) by overcoming the sedimentation problems associated therewith. However, never before have zinc powder and uncoated glass microspheres been incorporated together in a composition for a cathodic coating. The reasons for this complete lack of prior art in this respect are clear: (1) the extremely low specific gravity of the microspheres; (2) the low electrical conductivity of the microspheres; and (3) the large particle size and concomitant sedimentation problems associated with zinc powder.
In fact, the light, low electrical conductivity microspheres complement the heavy, high electrical conductivity zinc powder very nicely when combined, yielding compositions with the "best of both worlds", as demonstrated by the present application. Additionally, the low cost microspheres reduce the cost of coating containing zinc powder (which is expensive), again proving to be an excellent "foil" for the zinc powder, when combined. Some of the properties afforded by the microspheres and the zinc powder doubly enhance that property in the combination compositions of the present invention, such as adhesion.